I see what Christophe is saying, and he's quite right. The Textgrid system is not displaying the "header" information (= etext metadata = TEI header) in an appropriate manner. It makes it appear that Gruenendahl personally typed all the texts in GRETIL.
As to using services like Gitlab and Github, it can be a bit daunting at first. The whole Git ecosystem is fascinating and powerful, but not everybody needs it. And using it casually if you're not really into it is indeed daunting. In the case of Teodore's copy of GRETIL, you can get all of it by clicking this link:
and then pressing the blue 
button at the top right of the screen and choosing your preferred download format (probably Zip).
Best,
Dominik